This application claims priority to Japanese Patent Application Nos. H11(1999)-81042 filed on Mar. 25, 1999 and H11(1999)-283212 filed on Oct. 4, 1999, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to an image forming apparatus capable of forming an image on both sides of a recording sheet, such as an electrophotography type image forming apparatus, an electrostatic recording type image forming apparatus, or the like. More particularly, the present invention relates to an image forming apparatus preferably used as a color copying machine, a color printer, or the similar apparatus, equipped with an image-transfer belt or an image-transfer drum. The present invention further relates to a paper transferring apparatus used in an image forming method executed in an image forming apparatus or used for an image forming apparatus.
2. Description of Related Art
As shown in FIG. 10, in a Tandem-type color copying machine or the like, a recording paper supplied from one of paper-supply cassettes (trays) 101a, 101b, 101c, 101d by a paper-supply portion 101 is fed onto an endless image-transfer belt 105 by a timing roller 161. An image is formed on the recording sheet held and transported by the image-transfer belt 105 by an image forming portion 106. This image recording sheet transported by the image-transfer belt 105 is discharged to a recording sheet discharging portion 103 such as a sorter.
In some copying machines mentioned above, an inverting portion 109 is provided for inverting a recording sheet in order to form images on both sides of the recording sheet. In both-side image forming, the recording sheet on which an image is formed at one side thereof is transported from the image-transfer belt 105, and is inverted by the inverting portion 109. Thereafter, the inverted recording sheet is returned to the paper-supply side via a circulation path 110. Then, the recording sheet is again fed onto the image-transfer belt 105 by the timing roller 161 to form an image on the reverse side of the recording sheet.
However, in the aforementioned conventional color copying machine, in order to form images on both sides of the recording sheet, the recording sheet having an image formed at one side thereof transported from the image-transfer belt 105 is returned to the paper-supply side via the circulation path 110. Thus, it is required to provide the circulation path 110, which causes a large sized apparatus and a complicated structure, resulting in an increased manufacturing cost.
On the other hand, in the aforementioned Tandem type color copying machine which is equipped with an image-transfer belt and is capable of forming images on both sides of a paper as a recording sheet, it is known to provide a paper transferring apparatus equipped with a second absorbing device for absorbing a paper supplied from a paper supply-portion to an image-transfer belt and a first absorbing device for absorbing the paper having an image formed on one side thereof, which is inverted by an inverting portion, to the image-transfer belt in order to form an image on the reverse side thereof.
The aforementioned absorbing device supplies an electric current so as to pass through the paper, which causes electrostatic polarization of the paper and image-transfer belt. Thus, the paper is electrostatically absorbed by the image-transfer belt.
In a case where the image transfer-belt has a semiconductor characteristic (or a conductor characteristic), electric charges will leak as time passes to deteriorate the absorbing force.
However, in a conventional paper-transferring apparatus having a plurality of absorbing devices as mentioned above, the aforementioned leakage of the electric charges is not considered. As a result, the absorbing force will deteriorate during the transportation of the paper absorbed on the image-transfer belt by the upper stream side absorbing device before it reaches the downstream side absorbing device, resulting in a detachment of the paper from the image-transfer belt for the worst.
These problems occur not only in a paper transferring apparatus in which a paper is absorbed on and transferred by an image-transfer belt but also in a paper transferring apparatus in which a paper is absorbed on and transferred by an image-transfer drum.
It is an object of the present invention to provide an image forming apparatus which can assuredly form images on both sides of a recording sheet and is small in size and simple in structure.
It is another object of the present invention to provide an image forming method which is possible to execute in the aforementioned image forming apparatus.
It is still another object of the present invention to provide a paper transferring device which can keep enough absorbing force for holding a sheet, such as a recording sheet, absorbed and transported by a rotating member including an image-transfer belt and an image-transfer drum, until the sheet reaches the next absorbing means at the downstream side.
According to a first aspect of the present invention, an image forming apparatus, comprising:
a transferring device equipped with an endless rotating member for transferring a recording sheet while holding the recording sheet on a surface of the rotating member by rotating the rotating member, the transferring device having a first position, a second position and a third position in this order along a rotation path of the rotating member;
an image forming portion for forming an image on the recording sheet being transferred from the first position to the second position by the transferring device, the image forming portion being provided between the first position and the second position so as to face the transferring device; and
an inverting portion for receiving the recording sheet having an image formed by the image forming portion and transferred to the second position by the transferring device, inverting the recording sheet, and feeding the inverted recording sheet to the transferring device at the third position.
In this image forming apparatus, after forming an image on one side of the recording sheet transferred from the first position to the second position by the transferring device, the recording sheet is inverted at the inverting portion and fed to the transferring device. Therefore, it is not required to provide a circulation path or the like for directly returning the recording sheet having the image on one side thereof to the first position, resulting in a reduction of parts of the apparatus.
According to a second aspect of the present invention, an image forming method for forming images on a first side of a recording sheet and a second side which is a reversed side of the first side, the method including the steps of:
a first transferring step for transferring the recording sheet from a first position to a second position on a rotation path of an endless rotating member while holding the recording sheet on a surface of the endless rotating member rotatably driven in a state that the second side is contacted and held on the surface of the endless rotating member;
a first image forming step for forming an image on the first side of the recording sheet being transferred from the first position to the second position at the first transferring step;
an inverting step for detaching the recording sheet having the image formed on the first side at the first image forming step from the rotating member at the second position, and inverting the recording sheet;
a holding step for feeding the recording sheet inverted at the inverting step to a third position on the rotation path of the rotating member, and holding the recording sheet on the rotating member with the first side contacted the surface of the rotating member;
a second transferring step for transferring the recording sheet held on the rotating member at the holding step from the third position to the first and second positions in accordance with a rotational movement of the rotating member; and
a second image forming step for forming an image on the second side of the recording sheet being transferred from the first position to the second position at the second transferring step.
According to this image forming method, the recording sheet having an image on one side thereof is inverted, and the inverted recording sheet is held by the rotating member at the third position. Thereafter, the recording sheet is transferred to the first and second positions for forming an image on the other side of the recording sheet. Accordingly, it is not required to directly return the reversed recording sheet to the first position.
According to a third aspect of the present invention, a paper transferring device, comprising:
an endless rotating member;
a first absorbing device disposed on a rotation path of the rotating member for absorbing a sheet to the rotating member;
a second absorbing device for absorbing the sheet absorbed by the first absorbing device to the rotating member again on the rotation path, the second absorbing device being disposed at a downstream side of the first absorbing device; and
a controller for adjusting absorbing force to be generated by the first absorbing device depending on a sheet transferring time required to transfer the sheet from the first absorbing device to the second absorbing device.
According to the paper transferring device, the absorbing force to be generated by the first absorbing device is controlled depending on a sheet transferring time required to transfer the sheet from the first absorbing device to the second absorbing device. Therefore, enough absorbing force can be maintained within the sheet transferring time, in other words, until the sheet reaches from the first absorbing device to the second absorbing device. As a result, a stable paper transferring can be performed.